2019MNRAS.482..743R

We compare the rise and decay time-scales of ∼200 long-term (∼weeks-months) GeV and R-band outbursts and ∼25 short-term (∼hour-day) GeV flares in a sample of 10 blazars from the Fermi-LAT and the Yale/SMARTS monitoring programme. We find that most of the long-term outbursts are symmetric, indicating that the observed variability is dominated by the crossing time-scale of a disturbance. A larger fraction of short-term flares are asymmetric with an approximately equal fraction of longer and shorter decay than rise time-scale. We employ the MUlti-ZOne Radiation Feedback (MUZORF) model to interpret the above results. We find that the outbursts with slow rise times indicate a gradual acceleration of the particles to GeV energy. A change in the bulk Lorentz factor of the plasma or the width of the shocked region can lead to an increase of the cooling time causing a faster rise than decay time. Luminosity or the distance of the broad line region (BLR) affect the cooling time strongly if a single emission mechanism, e.g. external Compton scattering of BLR photons, is considered but may not if other mechanisms, e.g. synchrotron self-Compton and external Compton scattering of the torus photon, are included. This work carries out an extensive and systematic study, which can be used to estimate relevant geometric and physical parameters of the jet from the symmetry property of any observed short-term flares in the context of the MUZORF model.